This invention relates to carburetor metering systems for supplying a fuel/air mixture either alone or in combination with a wick or spark plug.
Automotive engines rarely operate at full power, so the part load condition is of greatest importance. This requires accurate metering of the fuel over a very wide range of flow rates. It has been said that the only reason a conventional carburetor can survive is because engines are very tolerant of rich mixture. Thus the conventional system, which cannot provide accurate metering over a wide flow rate range, is designed to provide richer than ideal mixture at operating points--such as low load--when it is not accurate. This approach is not adequate for today's conditions when emissions and fuel economy are subject to legislative control.
It is known that substantial advantages are to be obtained, in terms of part load fuel economy and decrease of exhaust pollution, by operating a spark ignition engine with a fuel/air mixture having excess air over that required for just complete combustion of the fuel, that is with a lean mixture of fuel in air. As mixture is weakened carbon monoxide emission rate falls rapidly to a low level and then remains low. NOx production is a maximum for air fuel ratios of about 17:1 (14.7:1 is chemically correct) after which it falls progressively. Unburned hydrocarbons fall progressively as mixture is weakened, down to a minimum after which they increase. This increase is caused either by very slow burning leading to flame extinction before completion or by occasional misfiring. Experiments show that improved ignition can hasten the whole combustion process to a degree and so postpone the increase of the leaner mixture. Improved ignition also minimizes the risk of misfire.
Conventional engines can readily tolerate excess fuel in the fuel/air mixture to a considerable degree. However, lean mixture operation requires precise control of mixture strength to ensure reliable operation without misfiring. Thus conventional carburetor systems are generally unsuitable for supplying engines operating at lean mixture strengths.
A carburetor system suitable for supplying lean mixtures of fuel in air is disclosed in British Patent Specification No. 1,595,315. This carburetor comprises an evaporator for evaporating the fuel into a stream of air and a closed-loop control arrangement for maintaining the mixture strength at a required value in dependence on the temperature drop measured across the evaporator. While such a carburetor is capable of operating adequately in a lean burn system, it has a fairly slow response time, typically of the order of 1/4 second, which can render the engine sluggish in operation. It would therefore be advangageous to provide a carburetor metering system which provides accurate fuel/air mixtures, particularly lean mixtures, and adjusts the mixture quickly in dependence on load changes over a wide range of air flows.
The present invention is also directed to an improved wick for a vaporizer usable in a carburetor metering system. One such wick vaporizer is disclosed in U.S. Pat. No. 4,290,401, in the name of the present applicant, and which is incorporated by reference herein. Such a wick vaporizor comprises a plurality of suspended wick elements having bottom ends of unequal length suspended above a liquid fuel reservoir, with the number of wicks that are wet at any one time being dependent on the level of fuel in the reservoir. Temperature measuring means are provided in an air stream both before and after the air stream passes through the vaporizer. The amount of measured temperature drop of the air stream across the vaporizer indicates the latent heat of the liquid fuel and thus the amount of liquid fuel being introduced into the air stream by evaporation. The level of fuel in the reservoir is adjusted in a controlled fashion in response to the temperature drop. This wick vaporizor arrangement provides good closed loop control. However an improved wick construction which provides an improved vaporization rate would be desireable.
The present invention also relates to a spark plug for use in a combustion system. A conventional spark plug has a body, within which is supported an insulated central electrode and on which a side electrode is secured so as to extend over the end of the central electrode. Conventionally the central and side electrodes have respective opposing flat surfaces, across which the spark is generated, the distance apart of these surfaces being adjusted by bending the side electrode towards or away from the central electrode. The two generally flat surfaces are thus often set at an arbitrary angle so that one portion of the pair of surfaces is closer together than another. The point or points at which the spark will occur is thus quite random.
Sparking, therefore, often occurs in a zone which is relatively enclosed between the electrodes and to which only a small amount of combustible mixture may have penetrated. Flame generation may thus, in some cases be relatively slow. This may give rise to incomplete combustion or poor combustion characteristics, in relation to the travel of the piston, or otherwise. Erosion of the electrodes due to uneven sparking, or sparking in one particular zone is also observable.
The time cycle is of course short but it has been shown that there are two stages of combustion, namely spark initiation, during which there is no pressure rise, and then general flame propogation across the combustion chamber. The boundary between the two stages is indistinct but can be defined as the point in time at which the pressure first departs detectably from the level it would have if no combustion took place. The duration of the first stage is known to depend on the pressure and temperature of the charge just before the spark and also on the mixture strength in and around the spark gap, but not, to any great extent, on turbulence in the combustion chamber. The duration of the second stage depends again on pressure, temperature and mixture strength but also very strongly on turbulence. Engines are usually timed so that, at a usual working speed, the first stage and about half of the second stage are completed by the time the piston reaches top dead center. Any significant variation from this timing results in loss of efficiency.
It is also well known that fuel economy and improved exhaust pollution are improved if the mixture is as lean as possible. The shorter the first stage, the more practical is the use of leaner mixtures.
When conventional sparking plugs are used, the duration of the first stage of ignition increases greatly as mixture strength is reduced.
For example, in the publication, "The High Speed Internal Combustion Engine", by Sir Harry Ricardo published 1953 (4th edition), results of tests are given as follows:
______________________________________ Angle turned by Mixture strength crank during first (% chemically correct) stage ______________________________________ 70 50.degree. 80 20.degree. 90 10.degree. 100 6.degree. 110 5.degree. 120 5.degree. ______________________________________
(These results were taken at an engine speed of 2000 rpm.) Increased spark advance reduces the pressure and temperature at time of the spark. As mixture is weakened or engine speed is increased the tendency of reduced pressure and temperature to increase the first stage duration as ignition is advanced eventually exceeds the advance and no further reduction in strength or increase in speed is possible.
It can be shown that the size of the opposing surfaces of conventional electrodes contributes to the relatively slow completion of the first stage of combustion, since the relatively large areas of metal contribute a cooling effect on the burning mixture. Indeed it can be calculated that the duration of the first stage approximately matches the time for the flame front to reach the limit of the electrodes.
U.S. Pat. No. 4,465,952 discloses a spark plug in which the central and side electrodes define respective parallel, elongate sparking surfaces arranged opposite one another. The sparking surfaces are coated with a noble metal and are flanked along their longitudinal edges by oppositely inclined surfaces sloping away from the sparking surfaces. However, such a spark plug is expensive to produce and does not satisfactorily solve the problem of igniting weak mixtures.
U.S. Pat. No. 4,122,816 discloses a spark plug having a central electrode having a frustoconical end whose outer curved surface constitutes a first sparking surface, and an annular side electrode surrounding the central electrode and having an inner surface of frustoconical shape inclined in the opposite direction to the frustoconical end of the central electrode and defining a second sparking surface opposite the first sparking surface. The spark gap of such a spark plug is generally annular and diverges towards the end of the central electrode. Whilst such an arrangement of the sparking surfaces will tend to cause the spark to advance towards the end of the sparking surfaces during ignition, the spark will tend to be confined by the annular shape of the side electrode. Furthermore the shape of the sparking surfaces will tend to result in wear of the surfaces in operation resulting in uneven sparking.
In view of the foregoing it is an object of the present invention to provide a carburetor metering system capable of accurately controlling fuel flow over a wide range of air flows and mixture strengths.
It is another object of this invention to provide a spark plug for use in a combustion system in which the effect of weak mixture on the duration of the first stage is minimized, this enabling the known advantages of lean mixtures to be realized, affording favorable combustion characteristics and fuel economy as well as minimizing pollution from unburnt or incompletely burnt gases.
It is another object of the present invention to provide an improved wick construction which provides an improved vaporization rate.